Solder bonding of packaged electronic chips to interconnection substrates is prevalent in the electronics industry. The most widely used forms of solder bonds are ball or bump bonds typically in a flip-chip assembly operation. The solder bonds form as pillars or beams that offset the joined surfaces with respect to one another leaving a gap therebetween. After the chip package is bonded to an interconnection substrate the resulting assembly typically undergoes further thermal cycling during additional assembly operations. The final assembly also is exposed to wide temperature changes in the service environment. The chip package is typically plastic, the interconnection substrate may be epoxy, ceramic or silicon. Both the material of the chip package, or the material of an intermediate interconnection substrate in the case where the solder bond interconnects interconnection substrates, frequently have thermal expansion coefficients that are different from one another, and are also different from the thermal expansion coefficient of solder. The differential expansion that the assembly invariably undergoes results in stresses on the solder bonds which can cause stress cracking and ultimately failure of the electrical path through the solder bond. To avoid solder bond failures due to mechanical stress, the gap between the surfaces joined by the bond is typically filled with an underfill material. The underfill provides additional bonding for the assembly and also protects the gap from moisture and other corrosive contamination. Underfill materials are adhesive and water insoluble. They are typically epoxy materials.
The underfill is applied after completion of the solder bonding operation. The underfill material is typically a polymer and is applied as a prepolymer liquid. Consequently, the viscosity of the underfill material as it is dispensed into the gap can be relatively low. The liquid prepolymer flows freely into the gap and, due to the relatively small gap in state of the art packages, flow is aided substantially by surface tension, and the liquid prepolymer is "wicked" into the gap. However, in some cases entrapped air, or incomplete wetting of the surfaces of the space being filled, inhibits flow or prevents wicking, causing voids in the underfill. Special problems arise with some package designs, e.g. microbumped packages, in which the solder bump pitch and the bump height is so small that the small gap resulting after the bonding operation cannot be filled consistently by relying only on fluid dynamics, and voids and bond failures are frequent.